The known devices for forming joints of cables are of the type in which at least a tubular covering, made of an electrically insulating material for at least a part of its thickness, surrounds the connection between the conductors and elastically tightens on the end portions of the cables which face each other in the joint.
Examples of known devices are described in the European Patent No. 149,032, in the German Patent No. 3,001,158 and in the Japanese Patent No. 54-49588.
All the known devices comprise a rigid tubular support on which a tubular covering for joints of cables is fitted in an elastically expanded condition and means for removing the support from the tubular covering in order to enable this latter to shrink elastically to embrace and tighten, always elastically, on the end portions of the two cables, which face or are adjacent each other, and thereby enclose the electrical and mechanical connection of the conductors of said cables.
The known devices have drawbacks which are linked to the above indicated known particular element of the devices themselves.
The known devices for forming joints have the diameter of the inner bore of the tubular support only a little greater than that of the outer diameter of the cables where the joints are to be formed so as to impose a stretching as small as possible, in the radial direction, on the elastically expansible tubular covering in order to avoid permanent plastic deformations which would prevent the tubular covering, once it is fitted on the connection between the conductors of the cables, from exerting the required tightening on the end portions of the cables.
The need of avoiding permanent plastic deformations in the tubular covering for joints of cables is in fact an essential need since if in a joint between cables the tubular covering enclosing the mechanical and electrical connection between the conductors of the cables has suffered a permanent plastic deformation in radial direction, the covering is no longer able to exert a sufficient compression on the end portions of the insulations of the cables embraced by said tubular covering. In these conditions, the joint runs the risk of becoming perforated under the action of the electric stresses applied to it during the use of the cable.
In fact, an insufficient radial pressure exerted by the tubular covering on the end portions of the insulations of the cables does not assure a complete absence of air bubbles or very small bubbles, between the surfaces in contact with each other and hence, does not assure a perfect tightness with respect to possible infiltration of traces of moisture between said surfaces.
Such requirements create difficult conditions for the above-mentioned elements of the devices for forming joints of cables, for the joints of cables, or for the tubular coverings of the joints due to the need, existing at present, for providing a plurality of rigid supports of diameters of different dimensions and related tubular coverings of diameters of different dimensions (to be fitted on the supports) in order to cover the wide range of diameters of the cables where the joints are to be made. The latter represents a drawback since it obliges the users to store a considerable number of sleeves and relative supports of different types in order to cover any possible situation.
To avoid risks of permanent deformation of the tubular coverings, the fitting of the coverings on the tubular supports is, at present, carried out immediately before effecting the joint. This is because a high elongation in radial direction maintained for a substantial period of time on a known tubular covering for joints fitted on a tubular support means results in the risk of causing in the covering a permanent plastic deformation in the radial direction which is greater the longer the time of application of said high elongation.
To effect the fitting of a tubular covering on a tubular support immediately before forming a joint means that the operation must be carried out in place with all the risks that this unavoidably involves because of the ambient conditions in the field which are difficult to control. Also, there is an unavoidable increase in the time for effecting the joints, and it is not possible to store the devices completely ready to apply to joints of electric cables.